The Federal Trade Commission yesterday ordered Equifax, Inc., the nation's largest credit reporting operation, to divest itself of two major credit bureau firms, including Washington's leading credit reporting agency.

In an opinion by Commissioner Elizabeth Hanford Dole, the FTC ruled that the company's acquisiton of three major credit bureau firms in the early 1970s - Credit Bureau Inc. of Washington, D.C., Retail Credit Association of Portland, Oregon Inc., and Credit Bureaus, Inc., a West Coast chain of 43 bureaus in Califonia, Oregon, Idaho and Washington - violated federal antitrust laws.

The three acquisitions may have substantially lessened competition in the credit reporting industry in the San Francisco Bay area, Washington, D.C., Portland, Ore., and Tacoma, Was., the unanimous commission said. Newest member Robert Pitofsky did not participate.

While the FTC ordered Equifax to sell the assets of the Washington firm and the West Coast chain, it allowed the firm to retain the Retail Credit Association of Portland and the portland office it acquired as part of the West Coast chain.

The commission said an order requiring Equifax to sell the Portland properties would create "a new monopoly." In order to "restore competition" in the Portland area, however, the FTC sadi Equifax must assist in establishing a new credit bureau by providing a suitable purchaser with copies of the most current files contained in its Portland operation for a three-year period.

Equifax, known as Retail Credit Co. until 1976, is in the business of gathering and selling information to business concerns for insurance, claims, credit, employment and marketing purposes. In Atlanta yesterday, a spokesman said the firm would "vigorously contest" the commission's decision - it can appeal through the courts - on grounds that it was based "upon conclusions which are contrary to the evidence, based upon unreliable data and wholly inconsistent with the realities of the credit bureau industry."

The commission's decision largely affirmed, but reversed in part, an initial decision rendered two-and-a-half years ago by Administrative Law Judge Montgomery K. Hyun.

In assessing the Washington acquisition, the FTC said Equifax acquired a 100 percent market share by buying the leading firm when it had the resources and incentives to establish a major credit bureau in Washington on its own. It already was "a significant, actual competitor in Washington" when it made its acquisition, the FTC said.